Process for the production of nitrous oxide



amh 11, 1193, J. H. CASTNER ET AL 291119276 PROCESS FOR THE PRODUCTION OF NITROUS OXIDE Filed June 25, 1936 Jepara/or James fifiarsfner William? 152G125! ATTORNEY INVENTORS Patented Mar. 15, 1938 UNITED STATES PATENT OFFICE PROCESS FOR THE PRODUCTION OF NITROUS OXIDE James B. Castner and William E. Kirst, Woodbury, N. J., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware The present invention relates to the manufacture of nitrous oxide by the heating of ammonium nitrate to its temperature of decomposition and more particularly to such a process in which improvement in yield, purity and uniformity of gas evolution is obtained.

The commercial methods of the prior art for the manufacture of nitrous oxide have generally comprised the heating of ammonium nitrate to its decomposition temperature, with the resulting formation of gaseous nitrous oxide and water, according to the equation As generally carried out, the method of operation consists in introducing a considerable amount of ammonium nitrate, for example 600 pounds, into a retort vessel either in solid lump form or as a molten material. Heat is then ap- 1 plied until the charge is raised to the decomposition temperature, for example 240 to 260 C. Since the reaction is exothermic, no further application of heat is ordinarily necessary when once it has started. Indeed, a certain amount of 5 cooling may be desirable from this point on in order to exercise the desired control over the decomposition velocity. As the process is usually carried out, it is customary to carry out the decomposition until 60 to 80% of the ammonium 30 nitrate has been decomposed. The process is then stopped and a fresh charge of ammonium nitrate is added to bring the amount of the material to the original quantity. Upon heating, renewed evolution of nitrous oxide will again 5 occur. Nitrous oxide is thus obtained as a decomposition product together with water, which latter material may be largely removed by condensation.

While the chemistry of this process is simple, 40 the method of operation must be carefully controlled for several reasons. In the first place It is essential that the nitrous oxide produced should be of high purity, since the principal demand therefor is as an anaesthetic. A further 5 important consideration is that the reaction should proceed smoothly since the decomposition reaction involves the conversion of a solid to gaseous products, a very great increase in volume occurs, and an uncontrolled reaction might, 5 therefore, become explosive in its effects. This control of the reaction velocity is important not only from the safety point of View, but also from the practical consideration. of yield. For the purity of the product is to a considerable extent dependent on the even course of the reaction,

since under some conditions of decomposition nitrogen gas and other impurities may be produced.

The explanation of these sudden semi-explosive increases in gas evolution which occasionally take place is not definitely known. Such occurrences are, of course, due to an increase in reaction rate and are generally accompanied by an increase in side reactions which give such undesirable products as ammonia, nitric acid, 10 the higher oxides of nitrogen, or nitrogen itself. Aside from the effect of local overheating, there is no general agreement as to the reasons for the occurrence of these violent reactions. It seems probable, however, that the presence of certain 15 impurities in small amounts in the reaction material exerts an undesirable catalytic effect on the various side reactions. For it is observed that the purity of the gas produced falls off steadily as fresh quantities of ammonium nitrate 20 are added to the heel. Apparently the impurity which exerts the undesirable catalytic effect is non-volatile and therefore increases progressively in amount as successive quantities of ammonium nitrate are added to the retort.

I-leretofore various attempts have been made to counteract the efiect of these occasional semiexplosive decompositions. These attempts, however, have not successfully overcome the cause of the difiiculty and manufacturers have contented themselves by providing means for rapidly cooling the mixture, pressure vents and releasable lids, so that the danger to life or permanent equipment would be minimized.

The object of our invention is an improved process for the production of nitrous oxide by thermal decomposition of ammonium nitrate. A further object is such a process characterized by improved yields, increased purity of product, and uniform velocity of gas evolution. A still further object is a continuous process for manufacturing nitrous oxide of high purity in high yields. Additional objects will be disclosed as our invention Is hereinafter more fully described.

We have found that the foregoing objects are accomplished by carrying out the reaction in the presence of the catalyst disclosed and claimed in our copending application, Serial No. 87,202 filed June 25, 1936, in such a manner that a substantially constant ratio is maintained between the amount of ammonium nitrate being decomposed and the amount of catalyst present in the reaction mixture.

In the copending case above referred to, a great many materials are disclosed which not only increase the rate of decomposition of ammonium nitrate to nitrous oxide, but also inhibit the side reactions which result in the formation of nitrogen and other impurities. Various compounds containing phosphorous, arsenic, molybdenum, silver, and manganese which exhibit this catalytic effect are therein disclosed. The phosphorousand arsenic-containing compounds are especially effective particularly water-soluble substantially non-volatile derivatives which contain these elements in the acid radical, for example arsenates and phosphates of ammonia and the alkali metals.

According to the present invention'adefinite improvement both in smoothness, of decomposition and purity of product is obtained by placing in the reaction vessel a suitable amount of the catalyst described and claimed in the above mentioned copending application. Preferably pure ammonium nitrate is then added to the retort and the mixture is heated until decomposition takes place. As'the ammonium nitrate in the retort decomposes, ammonium nitrate is fedto the vessel, either continuously or at short intervals, so that substantially the same quantities of ammonium nitrate and catalyst are present in the reaction vessel at all times. By operating inthis manner, the reaction may be carried out continuously or semi-continuously to obtain nitrous oxide of high purity in excellent yields.

In order to describe our invention more clearly, reference is made to the accompanying drawing which illustrates a preferred embodiment thereof. It is to be understood, however, that this is done solely by way of illustration and is not to be regarded as a limitation upon the scope of our invention.

The single figure oi the drawing represents a vertical section of a suitable apparatus for carrying out the process of our invention. The apparatus comprises a suitable reaction vessel l of metal, for example iron or aluminum, provided with a man hole 2, a gas outlet 3, and an ammonium nitrate inlet 4. Heat is supplied beneath the retort by means of the gas burner 5. The gas outlet 3 is connected through the condenser 6 to the separating chamber 1' whereinthe condensate is separated from the gaseous nitrous oxide. From the separating chamber 1 the gas proceeds through the line 8 to the compressor or storage tank (not shown).

In operation, 60 pounds of ammonium dihydrogen phosphate for example, and 540 pounds of ammonium nitrate are introduced into the retort l. Heat is supplied to the retort. by means of the gas burner 5 until the ammonium nitrate is brought to the temperature of decomposition. As the nitrous oxide and water vapor are evolved, they pass through the gas exit line 3 into the cooler condenser 6 where the water vapor is condensed. The condensedwater is separated from the gaseous nitrous oxide in the separating chamber 1 and the N20 thence passes through the line 8 to the storage tank or to the compressors. As the ammonium nitrate is decomposed, additional quantities of ammonium nitrate are added through the line 4 so that substantially the same volume of ammonium nitrate is maintained in the retort I throughout the course of the reaction. 1 V

The ammonium nitrate fed to the retort I may be added either in a solid or molten'condition, or as an aqueous solution, as desired. Preferably the ammonium nitrate is added continuously. Inview of the fact, however, that the catalyst is present in a small percentage, it is possible to maintain the ratio between the amount of catalyst and the amount of ammonium nitrate substantially constant, by operating in a semi-continuous manner, as for example, by adding the ammonium nitrate at suitable intervals, so that the quantity of material in the retort remains substantially constant. When operating in this manner, the quantities added and the intervals between successive additions will, of course, depend on the rate at which the reaction is proceeding.

, Since the ammonium phosphate is substantially non-volatile under these conditions there will be no loss of the catalyst. The particular advantage of this procedure, therefore, arises from the fact that substantially constant amounts of catalyst and ammonium nitrate are present in the reaction vessel l at all times. In other words, the ratio of the amount of catalyst to the amount of ammonium' nitrate is substantially constant. The rate of decomposition is therefore uniform and the nitrous oxide produced is characterized by an exceptionally high purity throughout the course of the reaction.

In the foregoing detailed description of our invention it is apparent that many variations in detail may be made without departing from the spirit and scope thereof. Thus, for example, any of the other catalysts described and claimed in ourcopending application above referred to may be employed in place of ammonium phosphate. Furthermore, the ammonium nitrate added to the reaction vessel may itself contain a small percentage of catalyst, for example.0.01%. The amount of catalyst added in this manner will, of course, not materially alter the ratio between catalyst and ammonium nitrate, due to the relatively large amount of catalyst (60 pounds) which was originally placed in the reaction vessel. is also apparent that our improved process for the efficient production of nitrous oxide makes no claims as to temperature limits which must be observed during the decomposition period, but simply requires heating to the temperature where the decomposition of the ammonium nitrate produces the desired gas generating rate. Other variations in detail will be apparent to any one skilled in the art. We, therefore, intend to be limited only by the following patent claims:

We claim:

1. The process for the production of substantially pure nitrous oxide by thermal decomposition of ammonium nitrate, which process comprises maintaining at the decomposition temperature a composition comprising ammonium nitrate and a catalyst comprising a compound containing an element selected from the group consisting of phosphorous and arsenic, and adding ammonium nitrate thereto at a rate suflicient to maintain a substantially constant ratio between the amount of catalyst and the amount of ammonium nitrate present in the reaction vessel.

2. The process for the production of substantially pure nitrous oxide by thermal decomposition of ammonium nitrate, which process comprises maintaining at the decomposition temperature of ammonium nitrate a composition comprising ammonium nitrate and a phosphate and adding ammonium nitrate thereto at a rate sufficient to maintain a substantially constant ratio between the amount of catalyst and the amount of ammonium nitrate present in the reaction vessel.

3. The process for theproduction of substan- -76 tially pure nitrous oxide by thermal decomposition of ammonium nitrate which process comprises charging into a reaction vessel a catalyst comprising a compound containing an element selected from the group consisting of phosphorus and arsenic, subsequently adding ammonium nitrate to said vessel, heating the resultant composition to its decomposition temperature, and adding to said reaction vessel ammonium nitrate at a rate substantially equal to the rate of decomposition of the ammonium nitrate present in said vessel.

4. The process for the production of substantially pure nitrous oxide by thermal decomposition of ammonium nitrate which process comprises charging into a reaction vessel a catalyst comprising a phosphate, subsequently adding ammonium nitrate to said vessel, heating the resultant composition to its decomposition temperature, and adding to said reaction vessel ammonium nitrate at a rate substantially equal to the rate of decomposition of the ammonium nitrate present in said vessel.

5. The process for the production of substantially pure nitrous oxide by thermal decomposition of ammonium nitrate which process comprises charging into a reaction vessel a catalyst comprising ammonium phosphate, subsequently adding ammonium nitrate to said vessel, heating the resultant composition to its decomposition temperature, and adding to said reaction vessel ammonium nitrate at a rate substantially equal to the rate of decomposition of the ammonium nitrate present in said vessel.

6. The process for the production of substantially pure nitrous oxide by thermal decomposition of ammonium nitrate, which process comprises maintaining at the decomposition temperature a composition comprising ammonium nitrate and a catalyst comprising a compound containing phosphorous, and adding ammonium nitrate thereto at a rate suificient to maintain a substantially constant ratio between the amount of catalyst and the amount of ammonium nitrate present in the reaction vessel.

'7. The process for the production of substantially pure nitrous oxide by thermal decomposition of ammonium nitrate which process comprises charging into a reaction vessel a catalyst comprising a compound containing phosphorous, subsequently adding ammonium nitrate to said vessel, heating the resultant composition to its decomposition temperature, and adding to said reaction vessel ammonium nitrate at a rate substantially equal to the rate of decomposition of the ammonium nitrate present in said vessel.

JAMES B. CASTNER. WILLIAM E. KIRST. 

